Samm 368 is a plasmacytoma of BALB/c mice previously shown to produce two classes of immunoglobulin which do not share idiotype. The tumor has been adapted to in vitro conditions and cloned. The majority of clones continue to produce both immunoglobulin classes (IgG2b and IgA) but variant clones producing only one class were detected. Cyanogen bromide fragments of the IgG2b heavy chain have been partially sequenced. The sequences define this protein as being an IgG2b inspite of the fact that it does not carry appropriate allotype determinants for this immunoglobulin class. NZB plasmacytomas with products reactive with antisera specific for IgA and IgG3 were studied. Both determinants of these reactions appear to reside on a single molecule.